I am thinking about joining the National Guard so I can get some experience, stay in shape and finish my schooling. Then go PJ. Which MOS should I choose?

The Prior service program is one of the Air Force’s most selective programs. We often have prior Army, Navy and Marines that go years without jobs. Even though they have all passing PAST tests. The Air Force’s priority is new enlisted for pipeline slots, then maybe backfill with AFR or ANG students, and then finally Prior Service. So I would really recommend not going in the Army in general, and just go straight in for PJ if that is what you ultimately want to do. The reason I say this is because there really is not a career field you can go into that is going to make you more prepared, PJs need to be good at all of that stuff including the medical and "ground pounder" aspects of the job. Additionally, you can still attend classes or just do online classes while you are in the Air Force. The biggest advantage you will gain from being in the Army is an appreciation for the Air Force to be honest.